It is known to connect to electric power networks static compensators, in shunt connection, for compensation of the power network and the reactive power consumption of equipment connected to the power network. One type of such compensators comprises at least one, and usually a plurality of, thyristor-switched capacitors. A thyristor-switched capacitor comprises essentially a capacitor in series connection with a controllable semiconductor. In addition thereto, usually an inductive element, an inductor, is arranged in series connection with the capacitor to limit the rate of change of the current through the capacitor at its connection to the power network and to avoid resonance phenomena with inductive components present in the power network. Such a thyristor-switched capacitor will hereinafter be referred to as a capacitor device and for a three-phase power network a compensator unit comprises three such capacitor devices, usually in .DELTA.-connection.
The controllable semiconductor valve comprises two controllable semiconductors, usually thyristors, in anti-parallel connection. By connecting the semiconductors, that is, by controlling their firing times relative to the phase position of the voltage of the ac network, the capacitor may be connected to the power network for generating reactive power. It is to be understood that, in this application, the concept capacitor also comprises those cases where the capacitor is composed of a plurality of interconnected capacitive elements, sub-capacitors, which are all commonly connected by the controllable semiconductor valve. Further, it is to be understood that, in this application, the concept semiconductor also comprises those cases when each one of the controllable semiconductors of the semiconductor valve is composed of a plurality of mutually series-connected thyristors, which are all commonly controlled by a firing order. A control device thus generates individual firing pulses for the semiconductors included in the semiconductor valve.
A compensator of the kind described above usually comprises a number of compensator units, the switching-in of which is controlled by a superordinate voltage control system which, in dependence on a sensed voltage in the power network and a reference value for this voltage, generates switching-in orders for switching in the respective compensator unit.
For a general description of thyristor-switched capacitors and control thereof, reference is made to, for example, Ake Ekstrom: High Power Electronics HVDC and SVC, Stockholm 1990, in particular pages 10-1 to 10-7, and to K. Reichert: Controllable reactive compensation. Electric Power & Energy Systems, Vol. 4, No. 1, January 1982, pages 51-61.
In the following, the term fundamental component for a voltage or a current in an electric power network means that component of the respective voltage or current which is of a frequency corresponding to the frequency of the power network; for example, a power network with the nominal frequency of 50 Hz has a frequency of, or at least near, 50 Hz.
Since the current through the thyristor-switched capacitor stationarily has a phase position of 90 electrical degrees before the voltage across the same, the two semiconductors of the semiconductor valve should be given firing orders alternately and at the times when the time rate of change of the fundamental tone of the voltage across the thyristor-switched capacitor changes signs from a positive value to a negative value and inversely. If the phase position of the voltage is defined such that its amplitude is zero at 0.degree. and increasing in a positive direction, under stationary conditions these sign reversals take place at the electrical angles 90.degree. and 270.degree.. When the above-mentioned time rate of change changes signs from a positive to a negative value, a firing order should be given to that of the semiconductors, the conduction direction of which coincides with the expected current direction in the interval to follow, that is, with the above-mentioned convention, in the interval of 90.degree. to 270.degree.. This conduction direction will be referred to hereinafter as the expected current-carrying conduction direction for the interval in question. When the mentioned time rate of change again changes signs, a firing order is given to the other semiconductor, the conduction direction of which coincides with the expected current direction in the interval which is then to follow, that is, with the above-mentioned convention, in the interval of 270.degree. to 450.degree..
When the generation of a firing order is caused to cease, for example in dependence on the voltage control system, the current through the semiconductor valve will cease at the next zero crossing of the current. The voltage of the capacitor thus remains at a level determined by the voltage of the power network when the current through the capacitor is brought to cease. When a firing order is again generated, according to the criterion mentioned above, and the voltage of the power network has remained unchanged, the switching-in of the capacitor takes place, in principle, without any transient phenomena in current and voltage.
When switching in the capacitor in those cases where a voltage difference exists between the voltage of the power network and the voltage across the capacitor, however, harmonic oscillations are initiated, which may entail abnormally high currents through the semiconductor and the capacitor. These oscillations influence currents and voltages at the point of connection of the compensator to the power network, and this influence may be regarded as a deterioration of the electrical quality of the power network.
A known phenomenon in connection with thyristor-switched capacitors is that a misfiring of a semiconductor at a time when the voltage across the capacitor has reversed polarity in relation to the voltage of the power network, because the current through the capacitor then grows very rapidly, leads to an increase in voltage which is harmful to the capacitor, and to abnormally high currents for the semiconductor.
A known way of generating a firing order is to continuously apply a firing order to both semiconductors when a switching-in order occurs and when a first firing order has been formed in accordance with the above-mentioned criterion. In that way, both conduction directions of the semiconductor valve are kept open for the current through the capacitor and the risk of misfirings is eliminated.
However, studies have shown that the above-mentioned harmonic oscillations when switching in the capacitor, when a voltage difference exists between the voltage of the power network and the capacitor, are damped very slightly, so the change of state to undisturbed operation becomes very long.
Also for those cases when the voltage of the power network contains harmonics, a resonance state may occur with detrimentally high currents for the thyristor-switched capacitor.
Another known way of generating a firing order is, at the beginning of each of the above-mentioned intervals of 180.degree., that is, at the times when the time rate of change of the voltage of the power network changes signs, from a positive to a negative value or inversely, to give a firing order only to that semiconductor, the conduction direction of which coincides with the expected current direction in the following interval. A firing order for the two semiconductors is thus generated alternately and for that semiconductor, the conduction direction of which coincides with the expected current-carrying conduction direction during the interval. A phase-locked loop forms, in dependence on the voltage of the power network, a sinusoidal signal such that it corresponds to the fundamental component of the voltage across the capacitor device and is phase-locked 90 electrical degrees before that voltage. This sinusoidal signal thus constitutes the time rate of change of a value of the fundamental component of the voltage across the capacitor device, which value is derived from the voltage sensed in the power network. A firing order for the valve is generated, in principle, at, but in practice shortly before, the zero crossings of the phase-locked signal.
With the above method for generating a firing order, a good damping is achieved of the transients after a switching-in when there is a voltage difference between the power network and the capacitor, and also of the above-mentioned resonance phenomena when the voltage of the power network contains harmonics.
Under certain conditions, typically in case of rapid changes of the phase position of the voltage of the power network, caused, for example, by a fault therein, because of the transient time of the phase-locked loop, however, at least a transient situation arises where the sinusoidal signal formed by the phase-locked loop is not phase-shifted 90 electrical degrees with respect to the voltage of the power network. A consequence of this is that the current through the current-carrying semiconductor approaches zero within one of the above-mentioned intervals of 180.degree. and is thereby caused to cease and remain zero until a firing order is generated for the reversed conduction direction. The voltage in the power network will thus be built up as an off-state voltage in the conduction direction across that semiconductor which is next to receive a firing order, which voltage, when the firing order is generated, leads to a high current through the capacitor with an ensuing risk of overvoltages thereon.